In the US, copyright inheritance is regulated by varying state laws. But, in general, if you have six children, you can give each child 1/6 interest in the copyright, either through your will, or by dying without one. Unlimited copyright would mean, for many books, copyright being split dozens of ways within three or four generations. Within a millennium, some books could be split a billion ways. So I'm going to say that any argument for unlimited copyright is a poor one due to practical considerations.